This chapter discusses acute abdomen, defined as severe abdominal pain lasting ≤24 hours, which may necessitate urgent surgical referral. The underlying cause can be intra-abdominal, thoracic, or systemic in origin and may result from infection, inflammation, vascular occlusion, or obstruction. Patients typically present with a sudden onset of abdominal pain, often accompanied by symptoms such as nausea, vomiting, constipation, diarrhea, haematemesis, melaena, dysuria, or diaphoresis. Alarm features that may indicate a surgical emergency include fever, persistent vomiting, syncope, or signs of gastrointestinal bleeding. Clinicians must assess for immediately life-threatening conditions such as a ruptured abdominal aortic aneurysm, bowel perforation, or ruptured ectopic pregnancy. Other critical differential diagnoses that must not be missed include myocardial infarction, pulmonary embolism, and pneumonia.